The invention relates to superconducting ceramic tapes, and particularly to laminated superconducting ceramic tapes.
It is known to form superconducting ceramic conductors by laminating the superconductor tape to a support structure to improve the tape""s tolerance to tensile and bending stresses. However, when an article containing superconductor tape is immersed in a liquid cryogen, any contact between the cryogen and the superconducting ceramic, whether at an exposed surface of the tape or through defects in the matrix surrounding the superconducting material, allows the liquid cryogen to enter the porous ceramic structure. When the article is quickly warmed, which is standard practice in the operation of superconducting systems, the liquid cryogen trapped in the interstices of the ceramic expands quickly, creating xe2x80x9cballoonsxe2x80x9d in the matrix and damaging the intragrain bonds in the ceramic. This results in decreases in the mechanical strength and critical current carrying capacity of the article. Typically this is true even of laminated tapes, as the cryogen seeps into the gaps between the superconducting tape and the support structure. Thermal cycling, which is a necessity for the normal operation of many superconducting articles increases this effect. To address this issue, it has been known to xe2x80x9cpotxe2x80x9d certain superconducting articles, such as small coils with thick layers of epoxy to minimize the likelihood of contact between the liquid and the superconducting tape, or to use other forms of cooling, such as conduction cooling when the articles cannot reasonably be protected in this manner. However, for many applications, including transmission cables, transformers, and high field magnets, the preferred cooling method requires direct thermal contact of a liquid cryogen with the superconducting conductor. In these applications, conduction cooling is typically not adequate and applying a heavy epoxy coating is typically not feasible for an number of reasons which may include, for example, packing factor and flexibility requirements as well as the need to assure good heat transfer between the article and the liquid cryogen.
A high performance superconducting ceramic article for use in direct contact with a liquid cryogen bath is provided. It includes a superconducting ceramic tape having at least one surface vulnerable to cryogenic infiltration is sealed on each vulnerable surface to a non-porous metallic laminate, which also provides the desired support structure, in substantially impervious relation by a non-porous metallic bonding agent. This results in greater protection of the superconducting ceramic tape from cryogenic infiltration, provides a cooling path for the superconducting article during the cool-down portion of a thermal cycle, and permits greater thermal cycling of the superconductor during use without causing degradation of the tape""s critical current carrying capacity. In some embodiments, it also improves the tape""s tolerance to tensile and bending stresses.
By xe2x80x9cnon-porousxe2x80x9d and xe2x80x9cimperviousxe2x80x9d are meant substantially non-porous and substantially impervious to one or more of liquid helium, liquid nitrogen, liquid hydrogen and liquid argon at the operating temperatures and pressures for which the superconducting article is designed. In particular embodiments of the invention, the laminate is a tape formed from stainless steel, copper, copper alloys, or superalloys. In particular embodiments of the invention, the laminate has a coefficient of thermal expansion within about 50% greater or less than that of the superconducting ceramic tape. Solder is the preferred metallic bonding agent. A first vulnerable surface of the superconductor tape can be sealed to one laminate and a second vulnerable surface of the superconductor tape can be sealed to a second laminate.
According to another aspect of the invention, a cryogenically cooled assembly comprises a liquid cryogen and a vessel for containing it, and a superconducting article at least partially immersed in the liquid cryogen. The article includes a superconducting ceramic conductor in direct contact with the liquid cryogen which comprises a superconducting ceramic tape having at least one surface vulnerable to cryogen infiltration by the preselected liquid cryogen, and a non-porous laminate sealed to the vulnerable surface in substantially impervious relation. The. assembly may further comprising refrigeration means for cooling the liquid cryogen. According to additional aspects of the invention, particular articles including a superconducting coil and a cable conductor are formed from the superconducting ceramic tape of the invention.
According to another aspect of the invention, an apparatus for laminating a superconducting ceramic tape to a laminate includes means for cleaning the superconducting ceramic tape, a laminator and a feed guide for guiding the superconducting ceramic tape and the laminate along a laminate process path into the laminator. In particular embodiments, the laminator includes a dryer/heater for heating the tape and the laminate during lamination to expand the tape and the laminate and minimize voids in the bonding agent. In particular embodiments, the laminator includes a solder wave that flushes out voids in the bonding agent and a guide dam to control the thickness of the solder layers.
According to another aspect of the invention, a method of laminating a superconducting ceramic tape includes cleaning the superconducting ceramic tape to provide an improved bonding surface, feeding the superconducting ceramic tape through a laminator, feeding a metallic laminate into the laminator, and laminating the superconducting ceramic tape to the laminate with a non-porous metallic bonding agent. Preferably, the thickness of the seal joint is in the range of about 0.0001xe2x80x3 to about 0.001xe2x80x3, and most preferably about 0.0002xe2x80x3 to about 0.0006xe2x80x3.
In particular embodiments of the method of the invention the tape and the laminate are heated during lamination to expand the tape and the laminate and minimize voids in the bonding agent. In some embodiments, a second laminate is fed into the laminator and the superconducting ceramic tape is laminated to the second laminate.
Advantages of the system may include one or more of following. The superconducting ceramic tape is not subject to cryogen infiltration though surface defects or exposed surfaces during use that would result in degradation of the tape""s critical current carrying capacity.